Population differentiation in host‐plant use in a herbivorous ladybird beetle, Epilachna vigintioctomaculata

Herbivorous insects that are polyphagous as a species often exhibit monophagy or oligophagy at the population level (Fox & Morrow, 1981). The difference among populations in host-plant use may arise either intrinsically from genetic changes and/or extrinsically from changes in ecological factors such as availability of host-plant species (Rausher, 1982; Tabashnik, 1983; Hare & Kennedy; 1986; Scriber, 1986). Even when genetic changes are not involved initially, differential host use may lead to evolutionary divergence between populations. If local populations of a herbivore species use different host plants, the populations may diverge as each is subjected to natural selection for improved ability to use its own host species. Thus, variation in host-plant use between populations can reveal patterns of local adaptation and suggests the responses to past selective forces. Epilachna vigintioctomaculata(Coleoptera, Coccinellidae, Epilachninae) is a likely candidate in which to detect differential adaptation. Epilachna vigintioctomaculata is one of the closely related herbivorous ladybird beetles ( E. vigintioctomaculata complex) that has diversified greatly in external morphology and host-plant use in and around the Japanese Archipelago. Epilachna vigintioctomaculata mainly feeds onSolanum tuberosumL. (Solanaceae). Although some additional plants in the family Solanaceae are subsidiarily used by adult beetles, larvae develop almost entirely onS. tuberosum (Katakura, 1981). Some populations on Hokkaido (the main northern island of Japan) are found on Schizopepon bryoniaefoliusMaxim. (Cucurbitaceae) but this host is not attacked on Honshu (the main central island). Such variation suggests geographical variation in host-plant use (Katakura, 1981), as the populations on Hokkaido have depended and persisted on S. bryoniaefoliussince their discovery in 1951 (Katakura, 1981). Thus, the Hokkaido populations on S. bryoniaefoliusand Honshu populations may be subject to differential selection on their respective host-plants. Here we conduct reciprocal transplant experiments to test for growth performance differences between E. vigintioctomaculatacollected fromS. bryoniaefolius on Hokkaido andS. tuberosumon Honshu and determined whether the larvae from different populations exhibit differential adaptation to their own host-plant species.